Just joined, but figured I'd throw my two cents on the priest + mage.

I've done it, and found priest + pally + mage to be by far the best. The buffs you get from the pally on top of fast low mana heals makes it quite a nice combo.

Only a 3boxer myself, space is limited so can't see going 4 or 5 anytime soon. 4 70s though, and going for 8 on the same server.